1. Field of the Invention
The present invention relates to a capacitive sensor for detecting a physical quantity by using a capacitance formed between a movable electrode and a fixed electrode in a sensor element. For example, the capacitive sensor is used as an acceleration sensor, and can perform a sticking diagnosis.
2. Description of Related Art
US 2005/0016273 A1 (corresponding to JP-A-2005-43309) discloses a capacitive acceleration sensor in a vehicle. In the sensor, a capacitance between a movable electrode and a fixed electrode in a sensor element is varied by acceleration. The capacitance is converted into an electrical signal by a capacitance-voltage (C-V) conversion circuit, and the signal is differentially amplified by a signal processing circuit including a sampling and holding circuit, and a switched capacitor circuit. Thus, the sensor can output the amplified signal as a sensor output.
If a foreign object is stuck between the movable electrode and the fixed electrode in the sensor element, the acceleration may not be detected, because the capacitance variation corresponding to the acceleration may not be generated. Therefore, a sticking diagnosis is performed, in which the movable electrode is forcibly moved toward the fixed electrode so as to make the movable electrode in contact with the fixed electrode.
In the sticking diagnosis, a voltage for moving the movable electrode toward the fixed electrode is applied. The sensor output is different in cases in which the foreign object is stuck between the movable electrode and the fixed electrode or not. This is because a clearance between the movable electrode and the fixed electrode is different in the cases. Therefore, the foreign object can be detected based on the difference of the sensor output. For example, a carrier wave voltage (10V) is applied to the fixed electrode in a diagnostic mode, which is larger than the carrier wave voltage in a normal acceleration-measuring mode. Further, a reference voltage applied to an operational amplifier in the C-V conversion circuit is increased from Vdd/2 in the normal mode to Vpp in the diagnostic mode. Thus, a potential of the movable electrode is increased to Vpp so as to displace the movable electrode. Then, the sensor output is monitored.
However, the sticking diagnosis may be performed only in a small-acceleration detecting sensor used for detecting a relatively small acceleration. A large-acceleration detecting sensor used for detecting a relatively large acceleration requires a larger voltage for its sticking diagnosis, because a spring constant of the movable electrode is larger in the large-acceleration detecting sensor. In this case, a semiconductor element, e.g., transistor, included in the C-V conversion circuit and the switched capacitor circuit is required to withstand the larger voltage. Therefore, a size of a semiconductor chip in a sensor circuit may be enlarged against demands for miniaturizations. However, the large-acceleration detecting sensor is required to perform the sticking diagnosis without the enlargement of the semiconductor chip.